A prior art method consists of a sequence of separate die operations as shown in FIGS. 8-11. In those figures, the sheet metal parts only are shown. The dies that form the parts are not shown.
FIG. 8 illustrates the first die operation for drawing a flat piece of sheet metal into the form of a first part, such as an outer panel 80. The shape of the formed part is not shown since that form is irrelevant to this invention as the purpose here is to assemble the first part to a second part by hemming the edge of the first part. But, what is shown is the addendum 82 that is required to accomplish forming of the part in a drawing die and which must be trimmed off later.
FIG. 9 shows the addendum 82 being trimmed away from the material that actually forms the part 80. Next a flange 84 is formed along the edge of the part 80 as shown in FIG. 10.
Each of these three operations is performed in a separate die. The piece of sheet metal is moved into the first die, and then from die to die and finally out of the third die in the process.
Finally, the flanged panel 80 is positioned in a hemming die, as indicated by the phantom line flange 84 in FIG. 11, and a second part, such as an inner panel 86, is positioned in place on the first part 80. Then the hemming die folds the flange 84 into a hemmed edge 88, entrapping the inner panel 86 and securing it to the outer panel 80 to form a panel assembly 90.
Shortcomings of the prior art process are (1) it requires three forming dies and one hemming die, (2) the piece of sheet metal must be moved at least five times and (3) there are three opportunities for mislocation of the part in a forming or hemming die with resulting defective forming or hemming. Also, the prior art method requires a less desirable “rope” hem when the parts are made of aluminum sheet.
The technology for applying electromagnetic forces and the configuration of the coils exists in prior art. The forming and trimming of sheet metal with electromagnetics is believed to have been demonstrated.
Electromagnetic forming uses very high-current pulses in a specially designed electrical coil to generate magnetic fields, which impart opposing currents and magnetic fields in a highly electrically conductive metal workpiece, such as an aluminum alloy. With the coil held in a fixed position, the repulsive magnetic forces act upon the workpiece causing it to deform at very high strain rates. Metals deformed at these very high strain rates can exhibit “hyperplasticity,” a level of plastic ductility well beyond what the material is capable of during conventional forming, e.g., flanging and hemming operations.
There was a perceived need for developing a single station fixture and methods for trimming, flanging and hemming of panels into panel assemblies for vehicle doors and other closures. The development of apparatus and methods for applying electromagnetic forces to the trimming, forming and hemming of panel assemblies was also desired.